In the field of liquid crystal display devices, lateral electric field schemes such as IPS (in-plane switching) and FFS (fringe field switching) have been receiving attention in recent years. Among the liquid crystal display devices using a lateral electric field scheme, in IPS the pair of electrodes arranged on the TFT (thin film transistor) substrate do not overlap in a plan view, whereas in FFS the pair of electrodes do overlap in a plan view. In these liquid crystal display devices using a lateral electric field scheme, liquid crystal is held between a pair of substrates that are arranged facing each other, and a pair of electrodes are disposed on one of the substrates, the TFT substrate, separated by an insulating film, with the electric field for driving the liquid crystal being generated between this pair of electrodes.
In an FFS liquid crystal display device, the pair of electrodes are arranged on the TFT substrate above/below one another with an insulating film therebetween, with one of the electrodes being a pixel electrode and the other being a common electrode. The liquid crystal display device can be operated regardless of whether the electrode disposed lower among the pair of electrodes is the pixel electrode or common electrode. In this type of liquid crystal display device, there are proposals to use an oxide semiconductor film as the material for forming the channel region of the TFTs. An oxide semiconductor film has a higher electron mobility than amorphous silicon or the like, which would make it possible to operate the TFTs at high speeds.
A configuration is known whereby, in an FFS liquid crystal display device where the channel regions of the TFTs are formed by an oxide semiconductor film in this manner, a portion of the oxide semiconductor film has the resistance thereof lowered by a hydrogen plasma treatment or the like, and this lowered-resistance region is used as the conductive region constituting the pixel electrode or common electrode. Patent Document 1 below, for example, discloses an FFS liquid crystal display panel in which, among the pair of electrodes, the pixel electrode is disposed lower and the common electrode is disposed higher. In this liquid crystal display panel, a reducing insulating film is formed on the oxide semiconductor film, thereby reducing and lowering the resistance of the region of the oxide semiconductor film contacting the insulating film, and this lowered-resistance region functions as the pixel electrode. This cuts manufacturing costs by forming the lowered-resistance region out of a portion of the oxide semiconductor film, rather than performing a special treatment such as a hydrogen plasma treatment on the oxide semiconductor film.